Miniature vibrotactile actuators are used in a very large number of industrial applications, in particular in the field of haptic interfaces, which allow a human being to receive a piece of information transmitted by a machine by perceiving a tactile sensation.
Among those haptic interfaces, force-feedback haptic interfaces which are used in particular in virtual reality devices provide a user with a tactile sensation corresponding to the one which would be felt in a simulated environment by the device. There are a large number of applications: video games, driving or flight simulation, simulation of surgery, etc.
Other haptic interfaces are used only for transmitting a warning to a user, for example, in the case of a vibrating element of a mobile telephone or a tactile tablet, for example.
A major issue in the development of those applications is the miniaturization of the vibrotactile actuators. These actuators transform an electrical signal, which is generated by any machine (computer, mobile telephone, etc.), into a vibrating signal which is perceptible by touching it. That miniaturization must be accompanied by an amplitude of vibrations which is as great as possible. The frequencies intended are those of the tactile range, between 20 Hz and 1000 Hz.
When a permanent magnet technology is selected in order to produce a linear vibrotactile actuator (in place of a piezoelectric technology, for example), it is particularly advantageous to use a so-called “non-ferrous” actuator. Those actuators comprise a mobile element which does not have any device for channeling magnetic field lines and which comprises at least one permanent magnet, and a fixed element which comprises at least one electrical coil in which an electric current flows. A force produced by the electric current and by a magnetic field produced by the permanent magnet generates a linear movement of the movable element in relation to the fixed element.
The main advantages of this type of actuator are:                a reduced spatial requirement,        the great weight of the movable element in relation to the fixed element, which allows the production of great accelerations, and therefore powerful vibrations,        a small number of elements which form the actuator, which allows the cost thereof to be reduced.        
The miniaturization of this type of actuator poses a given number of technical problems. In particular, it is difficult to produce a uniform magnetic field which has great intensity without any device for channeling field lines.
In order to illustrate this difficulty, a vibrotactile actuator of the prior art is illustrated in FIG. 1. The vibrotactile actuator of the prior art comprises a tubular body 1 which defines a cylindrical cavity 2 having an axis X1. The body 1 comprises two electrical coils 3 of conductive wire which are arranged coaxially relative to the axis X1 and are offset axially in order to surround the cylindrical cavity 2.
The vibrotactile actuator of the prior art further comprises an axisymmetrical movable fitting 4 which is inserted with little play inside the cylindrical cavity 2 in order to be able to slide inside it. The movable fitting 4 substantially comprises a permanent magnet 5 whose outer casing defines the cylindrical shape of the movable fitting 4 and which has North (N), South (S) axial polarities so that the magnetic field generated by the permanent magnet 5 has cylindrical symmetry. The permanent magnet 5 is in this instance fixedly joined to two supports 6 which extend at one side and the other of the permanent magnet 5 in order to have ends 7 which are connected to ends 8 of the body 1 by resilient diaphragms 9 which form a return means of the movable fitting 4 towards a position of equilibrium inside the body 1 without any power supply from the electrical coils 3. The resilient diaphragms 9 further bring about the sealing of the inner side of the cylindrical cavity 2. Field lines 10 of the permanent magnet 5 in the plane of section are illustrated in FIG. 1. When the electrical coils 3 are supplied with an alternating electric current, Laplace forces are applied to the electrical coils 3, inducing opposing forces which act on the movable fitting 4 in order to bring about the alternating linear movement thereof in the body 1. The alternating accelerations to which the movable fitting 4 is subjected during its alternating movement produce the vibrations generated by the vibrotactile actuator of the prior art.
In the Figure, it may be noted that the field lines of the permanent magnet 5 are not mutually parallel when they extend through the electrical coils 3, so that the magnetic field generated by the permanent magnet 5 is not uniform beside the electrical coils 3. Furthermore, only some of the field lines 10 extend radially beside the electrical coils 3 so that the intensity of the electromagnetic force induced is not at a maximum.